I use Steam for Mac about a half of the year and about a week ago it's worked fine. But now after one quit or reboot it don't work until I redownload and reinstall it. After launching it's icon appears in Dock only for a second and then disappears. Can you help me with it? Mac OS X 10.8.3

You should now be able to see the Steam folder, and the registry.vdf file inside that. Simply rename the file, or delete the full stop so that Steam doesn't recognise the file (you can delete the file, but best to rename first as if this fails, you might want to be able to revert back).

Once the file has been renamed, you should be able to start up Steam and get back online. Note: if the above fails, try following all of the steps again but rather than starting up Steam, re-install it instead.